The present invention relates to a data reproducing apparatus and method handling a magneto-optical recording medium which has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer so formed that, in any region where the temperature of the magnetic film exceeds the Curie temperature of the switching layer, displacement of a magnetic wall is generated in the displacement layer to dimensionally enlarge the effectively recorded magnetic domain. More particularly, the present invention relates to a data reproducing apparatus and method capable of detecting generation of a magnetic wall displacement from a differential signal of a reproduced signal or a difference signal thereof in the time base direction and detecting data by the use of such detection output, hence performing proper reproduction of the data at a sufficiently low bit error rate even if any sudden DC level variation peculiar to a DWDD mode is caused in the reproduced signal.
There are known magneto-optical recording media employed as rewritable high-density recording media. And among such magneto-optical recording media, one type is attracting notice recently. It has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer, wherein a magnetic wall displacement of the displacement layer is generated in any region where the magnetic film temperature is rendered higher than the Curie temperature of the switching layer, so that the size of an effectively recorded magnetic domain is enlarged. A reproduction method handling such a magneto-optical recording medium is termed a DWDD (Domain Wall Displacement Detection) mode. According to this DWDD mode, a very large signal can be reproduced also from a tiny recording domain of a period below the optical limit resolution of a light beam, whereby a high density is attainable without the necessity of changing the wavelength of light, the numerical aperture NA of an objective lens and so forth.
Now a further detailed explanation will be given on such DWDD mode.
As shown in FIG. 9A, a magneto-optical recording medium 10 has a three-layer film of switched connection consisting of a displacement layer 11, a switching layer 12 and a memory layer 13 formed in this order. The memory layer 13 is composed of a perpendicular magnetizing film indicating a great magnetic wall reluctance. The displacement layer 11 is composed of another perpendicular magnetizing film indicating a small magnetic wall reluctance and having a high magnetic wall displaceability. The switching layer 12 is composed of a magnetic layer whose Curie temperature Ts is lower than those of the displacement layer 11 and the memory layer 13. Each arrow 14 in the individual layers denotes the direction of atomic spin. A magnetic wall 15 is formed in the boundary between regions where the atomic spin directions are mutually reverse.
If the surface of a recording film is locally heated by the use of a reproducing light beam (laser beam) 16, there is formed a distribution of temperature T as shown in FIG. 9B, and accordingly a distribution of magnetic wall energy density "sgr" is formed as shown in FIG. 9C. Since the magnetic wall energy density "sgr" generally becomes lower in accordance with a temperature rise, the distribution is such that the magnetic wall energy density "sgr" becomes minimum at the position of the highest temperature. As a result, a magnetic wall driving force F(x) for displacement toward the high temperature side, where the magnetic wall energy density "sgr" is low, is generated as shown in FIG. 9D. FIG. 9E shows the positional relationship between a spot 16P of the light beam 16 and a region 17 whose temperature is higher than the Curie temperature Ts of the switching layer 12.
In any area of the medium 10 where the temperature is lower than the Curie temperature Ts of the switching layer 12, the magnetic layers are mutually in switched connection, so that even when the magnetic wall driving force F(x) due to the above-described temperature gradient is applied, it is checked by the great magnetic wall reluctance of the memory layer 13 to eventually cause no displacement of the magnetic wall 15. However, in any area of the medium 10 where the temperature is higher than the Curie temperature Ts, the switched connection between the displacement layer 11 and the memory layer 13 is cut off, so that the magnetic wall 15 of the displacement layer 11 having a small magnetic wall reluctance is rendered displaceable by the magnetic wall driving force F(x) due to the temperature gradient. Consequently, upon entrance of the magnetic wall 15 into the connection cut-off region beyond the position of the Curie temperature Ts with scanning of the medium 10 by the light beam 16, then the displacement layer 12 begins to be displaced toward the higher temperature side of the magnetic wall 15.
Whenever any of the magnetic walls 15 formed at intervals corresponding to the recorded signal on the medium 10 passes the position of the Curie temperature Ts with scanning of the medium by the light beam 16, there occurs a displacement of the magnetic wall 15 of the displacement layer 11. Since the effectively recorded magnetic domain is enlarged dimensionally by such displacement, it becomes possible to reproduce a very large signal as well even from tiny recorded domains of a period below the optical limit resolution of the light beam 16.
As the light beam 16 scans the medium 10 at a fixed speed, the above-described magnetic wall displacement is generated at a temporal interval corresponding to the spatial interval of the recorded magnetic walls 15. The generation of such magnetic wall displacement can be detected as a change in the polarization plane of the reflected light of the light beam (laser beam) 16.
As shown by broken lines in FIG. 9A, a magnetic wall displacement is generated from the rear of the region 17 as well, so that the signal due to such magnetic wall displacement from the rear is superimposed as a ghost signal on the reproduced signal due to the magnetic wall displacement from the front. Although an explanation on this ghost signal is omitted here, the problem arising therefrom can be solved by properly contriving the application of a reproducing magnetic field or the recording film.
The DWDD type magneto-optical disk apparatus mentioned above is substantially similar in structure to any general magneto-optical disk recording/reproducing apparatus. FIG. 10 shows a partial structure of a conventional reproducing section in such an apparatus. A reproduced signal SMO obtained from an optical head (not shown) is supplied to an equalizer circuit 21 where the frequency characteristic thereof is compensated. A reproduced signal SMO, obtained after such frequency characteristic compensation is supplied to a binary coding circuit 22, which then converts the input signal into a binary signal S2.
The binary signal S2 outputted from the binary coding circuit 22 is supplied to a data detection circuit 23 and a PLL (phase-locked loop) circuit 24. In the PLL circuit 24, a clock signal CLK synchronized with the leading and trailing edges of the binary signal S2 is produced, and then the clock signal CLK is supplied to the data detection circuit 23. Subsequently in the data detection circuit 23, data are detected from the binary signal S2 by the use of the clock signal CLK and then are outputted as reproduced data PD.
On a DWDD type magneto-optical disk, signals are recorded in such a manner that data bit strings are first converted into, e.g., RLL modulated bits and then are processed in NRZI (Non-Return to Zero Inverted) mode where a portion with data inversion is expressed as 1 while a portion without data inversion is expressed as 0. In this case, the data detection circuit 23 converts, e.g., NRZI data into NRZ data, whereby RLL modulated data are obtained as reproduced data PD.
The binary coding circuit 22 consists of a comparator 22a having a fixed threshold value as shown in FIG. 11A, or a comparator 22b of FIG. 11B for integrating the binary signal S2 by an integrator 23 and feeding back the same to a threshold value, or a comparator 22c of FIG. 11C equipped with, on its input side, a DC controller 24 for calculating the envelope center value by the use of a peak hold circuit or a bottom hold circuit and then feeding back the center value.
In the above-described DWDD type, however, there still exist many problems to be solved. For example, some variation is caused suddenly in the DC level of the reproduced signal SMO. Such a phenomenon is supposed to be derived from that the direction of magnetization is indefinite and may be inverted with a certain probability in any region other than those contributing to detection of the signal by magnetic wall displacement in the light beam spot.
FIG. 12 graphically shows an actual reproduced waveform representing the above phenomenon. On a DWDD type magneto-optical disk, signal is recorded in the NRZI mode as described, and it is seen in this diagram that the DC level of the reproduced signal is suddenly varied upward in the portion indicated by an arrow P. Upon occurrence of such a phenomenon, it is impossible to obtain a proper binary signal in the binary coding circuit of FIG. 11A which converts the input signal into binary one with a fixed threshold value. Even in using the binary coding circuit of FIGS. 11B or 11C, the relevant DC level variations are extremely faster than the response speed of the coding circuit and may become continuous bit errors during a period prior to the follow-up of the coding circuit.
It is therefore an object of the present invention to provide a data reproducing apparatus and method capable of reproducing data at a sufficiently low bit error rate despite any sudden DC level variation caused in the reproduced signal.
According to one aspect of the present invention, there is provided a DWDD type data reproducing apparatus which includes a signal reproducing means for irradiating a light beam from the side of a displacement layer onto a magneto-optical recording medium while moving the light beam relatively to the medium, and displacing a magnetic wall which has a gradient on the magnetic recording medium in the direction of motion of the light beam spot and is formed in the displacement layer with a temperature distribution having a temperature region higher than the Curie temperature of at least the switching layer, thereby obtaining a reproduced signal which corresponds to the change on a polarization plane of the reflected light of the light beam; a magnetic wall displacement detection means for detecting generation of the magnetic wall displacement by the use of a differential signal of the reproduced signal obtained from the signal reproducing means, or by the use of a difference signal thereof in the time base direction; and a data detection means for detecting data by the use of the detection signal obtained from the magnetic wall displacement detection means.
According to another aspect of the present invention, there is provided a DWDD type data reproducing method which includes a first step of irradiating a light beam from the side of a displacement layer onto a magneto-optical recording medium while moving the light beam relatively to the medium, and displacing a magnetic wall which has a gradient on the magnetic recording medium in the direction of motion of the light beam spot and is formed in the displacement layer with a temperature distribution having a temperature region higher than the Curie temperature of at least the switching layer, thereby obtaining a reproduced signal which corresponds to the change on a polarization plane of the reflected light of the light beam; a second step of detecting generation of the magnetic wall displacement by the use of a differential signal of the reproduced signal obtained at the first step, or by the use of a difference signal thereof in the time base direction; and a third step of detecting data by the use of a detection signal which represents generation of the magnetic wall displacement detected at the second step.
In the present invention, every time any of the magnetic walls formed in the magneto-optical recording medium at intervals corresponding to the recorded signal passes the Curie temperature position in accordance with scanning of the medium by the light beam, the magnetic wall of the displacement layer is displaced, so that the level of the reproduced signal is changed sharply and suddenly in conformity with such magnetic wall displacement. Therefore, upon generation of the magnetic wall displacement, the level of the differential signal of the reproduced signal or that of the difference signal in the time base direction is raised. Consequently, generation of the magnetic wall displacement can be detected by the use of such differential signal or difference signal.
As the light beam scans the medium at a fixed speed, the aforementioned magnetic wall displacements are generated at a temporal interval corresponding to the spatial interval of the recorded magnetic walls. Therefore, it becomes possible to perform detection of data by the use of a detection signal representing the generation of such magnetic wall displacement. In this case, generation of the magnetic wall displacement is detected by using the differential signal of the reproduced signal or the difference signal thereof in the time base direction, so that detection of the displacement can be executed without being harmfully affected by any variation caused in the DC level of the reproduced signal. Thus, reproduction of the data can be performed at a sufficiently low bit error rate despite any sudden DC level variation of the reproduced signal peculiar to the DWDD type.
The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings.